Features and control factors of gentle-sloped fluvial sandbodies in rift basins: An example from the Wen’an Slope, Baxian Sag

doi:10.13745/j.esf.sf.2020.5.14

Abstract

Abstract:

The gentle-sloped fluvial sandbodies in rift lake basins are important oil and gas reservoir units. Based on core, well logs and seismic sedimentology, we analyzed the fluvial sedimentary system and the spatiotemporal distribution of gentle-sloped sandbodies in rifting lake basins to provide reliable basis for fine exploration of oil and gas. The results show that there are four NE-SW trending meandering-braided fluvial successions in the Dongying Formation, Wen’an slope, Baxian sag, consisting of channel, channel bar and floodplain subfacies, which can be further subdivided into six microfacies, including braided channel, meandering channel, sandy channel bar, muddy channel bar, crevasse fan and floodplain. The river spreads out 40°-65° to the north (angle (α)); the apparent width (l) of the river course is 1.47-2.64 km; the apparent width (w) of the main river course is 0.03-0.58 km; the measured thickness (H) of the river course belt is 16.0-52.0 m; the measured thickness (D) of the main river course is 8.0-23.0 m; the drilling interpretation thickness (d) of the main river course is 1-16.5 m; and the average thickness is 6.5 m. In the fault-depression transition period, the gentle-sloped fluvial facies were controlled by many factors, such as basin structure, climate, provenance, sediment compaction and velocity. Differential subsidence of rift basin led to unbalanced subsidence of basin, which was the main factor controlling the fluvial facies type and sandbody spatial distribution in the gentle slope zone. By influencing flow, periodic climate change controlled the river type, adjusting and transforming the early fluvial sediments, thence determining the combination of microfacies and sandbodies. The study also suggests that the development of thick mudstone upslope may limit hydrocarbon charging and migration, whereas the thick channel and channel bar deposited mid and downslope, cross-cut by faults and conduits for migration, most likely have reservoir potentials in the Bohai Bay Basin.

Key words: Baxian Sag, Dongying Formation, fluvial facies, sedimentary facies, sandbody distribution, quantitative characterization, sedimentary model

CLC Number:

ZHANG Zili, ZHU Xiaomin, LIAO Fengying, LI Qi, ZHANG Ruifeng, CAO Lanzhu, SHI Ruisheng. Features and control factors of gentle-sloped fluvial sandbodies in rift basins: An example from the Wen’an Slope, Baxian Sag[J]. Earth Science Frontiers, 2021, 28(1): 141-154.

Figures/Tables 14

Fig.1 Location of the Wen’an Slope, Baxian Sag, Bohai Bay Basin and the structure of the Paleogene Baxian Sag

Fig.2 Seismic reflection and lithology along the provenance direction of Ed3 in the middle section of the Wen’an Slope, Baxian Sag (Profile location shown in Fig.1a)

Fig.3 Sedimentary palaeogeomorphological map of Ed3 in the middle section of the Wen’an Slope, Baxian Sag (See Fig.1a for location)

Fig.4 The fluvial microfacies composition and lithologic characteristics of Ed in the middle part of the Wen’an Slope, Baxian Sag

Fig.5 Seismic reflection characteristics and stratigraphic division of the Wen’an Slope, Baxian Sag

Fig.6 Sedimentary facies, slice interpretation and lithologic-electrical characteristics of Ed3 in the middle section of the Wen’an Slope, Baxian Sag

Fig.7 Sedimentary facies of Ed3 in the middle section of the Wen’an Slope, Baxian Sag (See Fig.3 for location)

Fig.8 Stratigraphic slices and channel evolutionary characteristics of Ed3 in the middle section of the Wen’an Slope, Baxian Sag

Fig.9 Quantitative analysis of channel in the middle section of the Wen’an Slope, Baxian Sag (profile position shown in Fig.3). Adapted from [27].

Table 1 River channel parameters for Ed3 in the middle section of the Wen’an Slope, Baxian Sag

剖面	河道带宽度L/km	主河道带宽度W/km	河道带视宽度l/km	主河道带视宽度w/km	河道带厚度H/m	河道带平均厚度h/m	主河道带厚度D/m	主河道带平均厚度d/m	岩电解释河道厚度C/m	河道带宽深比P	主河道带宽深比p
a	1.1	0.12~0.27	0.72~0.86	78~210	16~32	22	12~23	18
b	2.25	0.16~0.75	1.47~1.75	104~583	24~43	32	10~20	15		45.9~54.7	6.9~35.9
c	3.15	0.06~0.65	2.01~2.45	39~505	17~51	34	12~25	18		59.1~72.1	2.2~28.1
d	2.5	0.05~0.55	1.63~1.94	33~427	19~45	35	8~22	16	4~16.5	46.6~55.4	2.1~26.7
e	2.45	0.03~0.58	1.60~1.91	119~451	17~43	33	8~18	13	1.5~8.8	48.5~57.9	1.5~34.7
f	3.4	0.04~0.41	2.22~2.64	26~318	20~52	39	6~14	8	1~4.4	56.9~67.7	2.6~31.8

Fig.10 Relationship among fluvial parameters for Ed3 in the middle section of the Wen’an Slope, Baxian Sag. Adapted from [27].

Fig.11 Controlling factors for and evolution of fluvial facies and vertical sounding analysis of single stage channels in Ed3 along the flow direction in the middle scation of the Wen’an Slope, Baxian Sag

Fig.12 Fluvial profile structure and lithologic characteristics of Ed3 along the flow direction in the middle section of the Wen’an Slope, Baxian Sag

Fig.13 Fluvial model of the Wen’an Slope, Baxian Sag. Adapted from [27].

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